AbstractObjectives. Since the second half of the twentieth century, with the development of structured diagnostic tools, population based studies on epidemiology of mental disorders are carried out. A special role is played by World Mental Health Survey Initiative which brings together a group of countries from different continents in order to carry out research projects according to standard methodology using the Composite International Diagnostic Interview. Polish EZOP study, which was conducted in accordance with the guidelines of WMH joined that group. The project was implemented under the Norwegian Financial Mechanism and the Financial Mechanism of the European Economic Area. Its aim was to estimate the prevalence of mental disorders in the Polish population of adults, assess the distance to mental disorders and to obtain knowledge about the perception of psychiatric disorders and treatment.Methods. The research tool was the Polish version of CIDI-WHO ver.3.0., which was developed for the project. The study was performed in accordance with the guidelines of WMH (cognitive interviews, interviewers training, standardization of field procedures) using electronic version of CIDI questionnaire (CAPI) within the 2-stages procedure in randomly selected representative sample of the Polish population aged 18-64. The quality of the study was systematically controlled and reported by MB SMG/KRC, and completed data (10,081 interviews) were sent to the Department-Centre of Monitoring and Analyses of Population Health NIPH-NIH. After the initial analysis data were sent to the analytical center of WMH, which applied additional cleaning tools and added new variables representing psychiatric diagnoses in DSM-IV and ICD-10.